In mobile communication systems, such as e.g. any 3GPP communication system beginning from the second generation (2G, 3G, 4G, and beyond), like UMTS, LTE, LTE-A, etc., efficient (i.e. appropriate and fast) handling of radio link problems including radio link failures represents a vital issue.
This is because such radio link problems including radio link failures can never be avoided entirely, and an appropriate and fast remedy thereof is required in order to ensure reliable communications. For such remedy, the connection could be tried to be recovered on its initial radio link (or channel), or the connection could be tried to be re-established on a new/alternative radio link (or channel). Since maintaining the initial radio link is less expensive in terms of processing/signaling load and/or latency, connection re-establishment is tried only when necessary, i.e. when the initial radio link has failed with a certain degree of reliability.
In conventional solutions, upon detection of a radio problem of a radio link carrying a network connection of a communication device (such as a UE), a recovery procedure for recovering the connection on said radio link is initiated. Only when the recovery procedure has failed to recover the connection on said radio link within a time period for detection/declaration of a radio link failure (RLF) in the recovery procedure (i.e. when a radio link failure has been detected/declared as a result of the recovery procedure), a re-establishment procedure for re-establishing the connection on an alternative radio link is initiated.
FIG. 1 shows a schematic diagram of an example of a conventional mechanism for radio link problem handling, as outlined above.
As shown in FIG. 1, when a radio problem is detected/declared in a normal operation (in which the UE has a network connection on a radio link to a network device of a serving cell), the UE initiates a recovery procedure. Specifically, the UE waits for a certain time (denoted as T310 in LTE) before it detects/declares a radio link failure. During this period, the UE has the chance to recover on the existing radio link. This is referred to as “first phase”. Only after this period, i.e. when the radio link failure is detected/declared, the UE starts a re-establishment procedure in order to look for an alternative radio link (to a network device of a new/target cell). This is referred to as “second phase”. When the re-establishment procedure is not successful either, the UE goes back to idle operation.
Herein, generally, connection recovery refers to (an attempt to) maintain the existing radio link (or channel) to the serving cell and to communicate on the existing radio link (or channel), and connection re-establishment refers to (an attempt to) establish/prepare/set-up an alternative/new radio link (or channel) to a new/target cell and to communicate on the new/target radio link (or channel).
A mechanism as shown in FIG. 1 is for example applied in LTE or LTE-A, wherein the normal operation, the radio problem detection, the recovery procedure and the re-establishment procedure (RRC connection re-establishment) are performed with the UE operating in RRC_CONNECTED mode, while the UE operates in RRC_IDLE mode in the idle operation.
Like the mechanism shown in FIG. 1, all conventional solutions clearly separate in time the first phase of connection recovery on the existing radio link and the second phase of connection re-establishment on an alternative radio link. That is, all conventional solutions assume that radio link failure detection/declaration (as a result of a failed recovery procedure) and connection re-establishment have to occur sequentially such that re-establishment actions are only started after the radio link failure has been declared. This is inevitable, as it is a direct consequence of typical assumptions for current mobile communication systems, especially in that the UE can only transmit data to and (coherently) receive data from a single cell, since it is only synchronized with a single cell.
Since connection re-establishment is more expensive then connection recovery in terms of processing/signaling load and/or latency, the radio link failure detection/declaration is to be very reliable to give the UE the chance to recover before the radio link is detected/declared to be failed. That is, connection re-establishment is to be avoided as far as possible, since connection re-establishment (including searching for a good cell, synchronizing with the good cell, starting a random access procedure to the good cell, starting actions to recognize (authenticate) the UE, and setting up the new connection to the good cell) is rather complex, which is basically due to the fact that the new (good) cell, i.e. the connection re-establishment destination, is not aware of or prepared for the intended UE access (handover). However, achieving increased reliability of radio link failure detection/declaration requires more time (in the recovery procedure) and thus delays the time when the connection re-establishment is started. On the other hand, in order to ensure reliable communications, the radio link failure is to be detected/declared very fast to give the UE the chance to quickly re-establish the connection on a new radio link.
Due to fast fading or interference fluctuation, it may happen that the serving signal disappears abruptly (such that there is no time for a proper handover) only for a short period and quickly returns. In those cases of a temporary radio problem, i.e. a pseudo radio link failure, maintaining the radio link would be the preferable option rather than detecting/declaring a radio link failure and starting the connection re-establishment procedure. On the other hand, in cases of true radio link failures, quickly detecting/declaring a radio link failure and starting the connection re-establishment procedure would be the preferable option rather than maintaining the radio link. Yet, the UE has no chance to distinguish these cases of pseudo and true radio link failures in good time.
Quantitatively, it can be approximated that reliable radio link failure detection/declaration requires waiting for a coherence time of the link (or channel) which is in the range of 200 ms (assuming 3 km/h as worst case). Starting the connection re-establishment procedure afterwards would lead to an interruption of 400 ms interruption in case of a “true failure” assuming that the connection re-establishment takes 200 ms. But there is still the chance to recover within the waiting time of 200 ms, so in many cases the interruption time would be much less than 200 ms.
In the other extreme case, no waiting time would be applied at all, meaning that each and every radio problem (even if it is very short) leads to an interruption of 200 ms which is required for the connection re-establishment. In case of a “moderate” waiting time of e.g. 50 ms, the interruption time in case of a “true failure” would be reduced to 250 ms, but the chance for an earlier recovery in case of a “pseudo failure” would be significantly reduced as well. Further, any of the above assumptions would not allow small latencies at a large level of reliability (“ultra reliable communication”), and it seems impossible to reduce the interruption/latency below approximately 50 ms which is a typical requirement in 5G systems (not even talking about the 1 ms requirement).
Even if the connection re-establishment procedure could be significantly reduced, there is still a conflict which does not allow for an optimal solution:                On the one hand, radio link failure detection/declaration should be reliable to avoid that the expensive connection re-establishment procedure is initiated unnecessarily. That is, the interruption time in case of a “pseudo failure” shall be shortened.        On the other hand, radio link failure detection/declaration should be as fast as possible to accelerate the recovery procedure and, thus, the start of the connection re-establishment procedure. That is, the interruption time in case of a “true failure” shall be shortened.        
As outlined above, conventional solutions for radio link problem handling assume that radio link failure detection/declaration (as a result of a failed recovery procedure) and connection re-establishment occur sequentially such that re-establishment actions are only started after radio link failure detection/declaration. Thus, conventional solutions suffer from a conflict of contradictory requisite in terms of reliability and latency of radio link failure detection/declaration.
Accordingly, there is a demand for enabling/realizing more efficient radio link problem handling in a mobile communication system.